The present invention refers to a sole for shoes particularly consisting of two preformed wooden parts being mutually connected at the area of the ball of the foot by means of an intermediate part.
Soles consisting of wood are already known. Such soles provide a comfortable feeling on walking and have the additional advantage that wood performs a humidity regulating action and thus absorbs excessive sweat and later releases the absorbed humidity. The stiffness of a sole consisting of wood is, however, particularly disturbing at the area of the ball of the foot, because, on walking, the feet shall only be supported at the area of their balls which is difficult, or even not possible, with a stiff sole consisting of wood.
For this reason, soles were proposed consisting of two preformed wooden parts which were mutually connected at the area of the ball of the foot by means of rubber reinforced by textile fabric (U.S. Pat. No. 1,964,364). Production of such a sole is quite laborious and creates a problem in connecting the intermediate part consisting of reinforced rubber, on the one hand, and both wooden parts, on the other hand. The intermediate part consisting of reinforced rubber must separately be produced in a corresponding mold and subsequently be connected with both wooden parts. The use of adhesives, Per se, do not result in the required secure connection of the intermediate part consisting of rubber with both wooden parts, so that dove-tail-shaped grooves and correspondingly shaped ledges must be provided and, furthermore, nails must be used in this type of connection.
The present invention has as an object to avoid the mentioned drawbacks and to provide a sole consisting of two wooden parts and a flexible intermediate part such that the intermediate part can be produced in a simple manner and can, simultaneously with its production, reliably be connected with both wooden parts. An essential feature of the invention is that the intermediate part consists of foamed polyurethane. When using this material for producing the intermediate part, a reliable connection between the wooden part and the polyurethane results without any additional measures because wood does chemically react with polyisocyanates contained within the polyurethane-forming mixture. Chemical compounds contained in the wood and comprising OH-groups react with their OH-groups with the isocyanate groups of the polyisocyanate and provide an unseparable connection. The required connection is thus formed simultaneously with the production of the flexible intermediate part, so that production of this intermediate part becomes possible in a substantially more economic manner than when separately producing the elastic intermediate part and subsequently connecting this intermediate part with both wooden parts by means of adhesives or in a mechanical manner by means of nails, screws, clips or the like. Furthermore, the polyurethane used has a lower specific weight than rubber, which also contributes to more economical production.
One way the upper of the shoe can be fixed to a sole according to the invention present for instance by pinching the upper over the sole. When using a sole according to the invention for sandals, clogs or the like it is, however, convenient to connect the upper to the sole by means of nails or clips. To achieve the required reliable connection between the upper and the sole, at the area of the intermediate part, there is, according to the invention, provided within the intermediate part at least one insert transversely extending relative to the longitudinal direction of the sole at least up to a lateral boundary of the intermediate part and being adapted to be connected with the upper of the shoe. Such an insert can, for instance, consists of a metallic tie pin or consists of another material such as synthetic plastics material or wood. It is, however, essential that the insert be well anchored within the intermediate polyurethane part and be in the position to provide a mechanically strong connection between the upper and the insert, so that the upper can reliably be connected to the sole also at the area of the intermediate part.
A separate insert can be provided at each of the lateral boundaries of the intermediate part, such inserts thus extending only over a certain distance into the interior of the intermediate part. However, it is convenient to provide, according to a further feature of the invention, at least one insert extending over the whole width of the intermediate part so that the upper can be connected to both ends of the same insert. In such an embodiment the insert is reliably prevented from becoming extracted out of the intermediate part because, if an extracting force is acting on one end of the insert, the other end of the insert would tend to be pulled into the intermediate part, which, however, is prevented by the upper fixed to this end.
According to a preferred embodiment of the sole of the present invention, the intermediate part has a substantially triangular cross section in longitudinal direction of the sole, noting that the base of this triangle, which is preferably an equilateral triangle, is flush with the walking surface of the sole. In such an embodiment, the area of the intermediate part adjacent the sole of the foot is narrow, whereas the area of the intermediate part adjacent the walking surface of the sole is broad. This provides the advantage that one can, when providing a connection with the upper, do with only on insert extending over the whole width of the intermediate part and being arranged within the narrow area of the intermediate part, because in this case the adjacent connecting points for the upper are located at the mechanically strong wooden parts of the sole. In such an embodiment, the connecting surfaces, between the intermediate part and both wooden parts are enlarged so that adhesion of the intermediate part to the wooden parts becomes improved. In this embodiment, the intermediate part is very broad at the area of the walking surface of the sole, so that the specific elongation per centimeter of the intermediate part can be kept low and the intermediate part does not become stressed to destruction by excessive elongation.
The tip of the triangle preferably merges into a lateral web within which the insert is provided. This permits secure anchoring of the insert even in an intermediate part having a triangular cross section.